The objective is to examine the variations in response of rats to porphyrogenic chemicals. Hexachlorobenzene was the chemical of choice, because it is an important fungicide and produces porphyria following ingestion for a few days. The porphyria reaches its peak a few months later and slowly declines. The pattern has been followed in individual rats. The existence of rats which do not respond by elimination of uro- or coproporphyrins in the urine has been detected. The animals were followed through their recovery period of 18 months after which time no evidence of porphyria remained. They were then given another porphyrogenic chemical, allyl-isopropylacetamide, (AIA) in the diet or by gavage to observe rapidly occurring and short-lived porphyria, resulting from this treatment. Variations in response of these rats to repeated application of this chemical were compared to their response to hexachlorobenzene. Explanations for the differences in response are being studied.